This invention relates to a wireless base station array antenna system. More particularly, the invention relates to an array antenna system such as an adaptive array antenna system in which a wireless base station of a DS-CDMA mobile communications system is provided with an array antenna having a plurality of antenna elements for converting signals received by the antenna elements to digital signals and combining the received signals upon subjecting them to any amplitude weight as well as phase rotation by arithmetic operations, thereby forming a desired beam pattern.
Digital cellular wireless communication systems using DS-CDMA (Direct Sequence Code Division Multiple Access) technology have been developed as next-generation mobile communications systems for implementing wireless multimedia communication. In such CDMA communications, transmission information from a plurality of channels or users is multiplexed by spreading codes that differ from one another and is transmitted via a transmission path such as a wireless link.
In wireless communications, radio waves from a transmitter arrive at a receiver via several paths (multipaths) having different path lengths. The receiver combines the radio waves. However, the combining of the radio waves is not performed by coherent addition, as a result of which fading occurs. Various diversity schemes have been proposed to deal with such fading. One example is a Rake receiving scheme. Rake reception is a technique which involves identifying signals that have passed through multipaths and combining the signals (by maximum-ratio combining) upon weighting them for reliability, thereby improving the characteristic. A receiver employing such Rake reception in CDMA communication has been proposed as a Rake receiver. FIG. 16A is a block diagram showing the construction of the prior-art Rake receiver, and FIG. 16B is a diagram useful in describing the delay profile thereof.
Shown in FIG. 16A are a searcher 1, fingers 2.sub.1 -2.sub.3 each of which is provided for a path of multipaths, an antenna 3, a Rake combiner 4 for combining the outputs of the fingers, and a discriminator 5 for discriminating the "1"s and "0"s of received data based upon the output of the combiner 4.
As shown in FIG. 16B, the reception level of the signal sent from a transmitter varies in the receiver in dependence upon the multipaths, and the times of arrival at the receiver differ as well. The searcher 1 (1) measures the profile of the antenna reception level (the temporal transition characteristic of the level), (2) detects the multipaths from multipath signals MP.sub.1, MP.sub.2, MP.sub.3, which are higher than a threshold level, by referring to the profile, (3) identifies delay times from signal occurrence times t.sub.1, t.sub.2, t.sub.3 of respective paths of the multipaths or from a reference time, and (4) inputs despreading start timings and delay-time adjustment data to the fingers 2.sub.1, 2.sub.2, 2.sub.3 that correspond to the respective paths.
The searcher 1 includes a matched filter 1a, which outputs the autocorrelation of a desired signal contained in the received signal. FIG. 16A illustrates the construction of one channel of a base station. That is, the reception output of the antenna 3 contains other channel components as well. The matched filter 1a uses the spreading code of its own channel to extract the signal component of its own channel from the antenna reception signal. The extracted signal component is delivered as the output. More specifically, when a direct sequence signal (DS signal) that has experienced multipath effects enters the matched filter 1a, the latter outputs a pulse train having a plurality of peaks conforming to arrival times and signal strengths and stores the pulse train in a RAM 1c via a low-pass filter 1b. A path detector 1d refers to the profile (FIG. 16B) that has been stored in the RAM 1c to detect each path constituting the multipaths as well as the delay times, and inputs the start signals, which indicate the timings (chip synchronization timings) of the start of despreading, as well as the delay time adjustment data, to the fingers 2.sub.1, 2.sub.2, 2.sub.3 corresponding to the paths.
The fingers 2.sub.1, 2.sub.2, 2.sub.3 corresponding to the respective paths are identically constructed and each includes a spreading code generator 2a for generating the spreading code assigned to its own channel, a multiplier 2b for multiplying the antenna reception signal by the spreading code to thereby despread the signal, a dump integrator 2c for performing dump integration, a delay time adjustment unit 2d for subjecting the despread signal to a time delay adjustment that conforms to the path, an arithmetic unit 2e which performs an operation for channel estimation, and a multiplier 2f for multiplying the input to the arithmetic unit 2e by the complex conjugate of the output thereof to compensate the fading channel and output a desired signal wave component corresponding to the channel. The complex conjugate is obtained by reversing the sign of the imaginary part of the complex number. If the complex number is I+jQ, then the complex conjugate thereof is I-jQ.
FIG. 17 is a diagram useful in describing the channel estimation and compensation operation. Shown in FIG. 17 is a transmitting antenna 3' of a mobile station, the antenna 3 of the base station, the arithmetic unit 2e that performs the operation for channel estimation of the finger, the multiplier 2f, and a complex conjugate arithmetic unit 2f' for outputting the complex conjugate. Let s represent a signal transmitted from the transmitting antenna 3' to the destination of the base station, .xi. the influence of the wireless path and r the reception output of the base station. The arithmetic unit 2e outputs the product rs* of the input signal r and desired signal s. Accordingly, the output of the arithmetic unit 2e is EQU rs*=s.xi.s*=.xi..vertline.s.vertline..sup.2 .varies..xi.
If there is no fluctuation in amplitude, the output of the complex conjugate arithmetic unit 2f' becomes .xi.*, and the output of the multiplier 2f becomes EQU r.xi.*=s.xi..xi.*=s.vertline..xi..vertline..sup.2 .varies.s
In other words, if the amplitude does not fluctuate, the signal s that was transmitted to itself is obtained from the multiplier 2f. Accordingly, the arithmetic unit 2e and multiplier 2f in FIG. 16A estimate and output the signal component of their own channel.
Thus, the fingers 2.sub.1 -2.sub.3 corresponding to the respective multipaths despread the corresponding multipath signals MP.sub.1 -MP.sub.3 by multiplying them by the spreading codes allocated to the channels and adjust the delays of the despread signals by the path delay times to make the timings agree. The Rake combiner 4 performs maximum-ratio combining of the finger outputs, and the discriminator 5 discriminates the received data based upon the output of the combiner.
Base station antennas of DS-CDMA communications system currently employ sector antennas. When the 360.degree. perimeter of an antenna is equally divided into a plurality of sectors, the antenna that is allocated to each sector is referred to as a sector antenna. Since there is no directionality within a sector, the antenna is susceptible to interference from other users. Such interference from other users is the main cause of a decline in channel capacity and transmission quality. Research and development in regard to adaptive array antennas is being carried out in an effort to discover techniques for reducing such interference and improving transmission quality. The adaptive array technique involves receiving signals by a plurality of antenna elements and combining the signals from each of the antenna elements upon optimally weighting the signals, thereby reducing interference signals on the receiving side.
Applying an adaptive array antenna (AAA) system makes it possible to obtain multiple beams within a sector and has the effect of raising gain by sharpening the beam patterns and of reducing interference in the area. As a result, it is possible to increase the number of users capable of being accommodated by a single cell or to improve communication quality.
A method indicated in the paper "Characteristic of Discrimination-Feedback Coherent Adaptive Diversity in DS-CDMA", TECHNICAL REPORT OF IEICE, RCS96-102 (1996-11) has been disclosed as an example of art in which AAA is applied to a wireless base station in a DS-CDMA mobile communications system. In order to raise signal precision by spreading gain, the conventional method (1) combines signals obtained by subjecting the signals received by the antennas to despreading processing, (2) then subjects the combined signal to Rake reception processing and performs data discrimination based upon the results of processing and (3) feeds back the results of discrimination and operates on the original signal to calculate an adaptive weight. With this conventional method, however, all elements of the DS-CDMA mobile communications system inclusive of the Rake receiver, which is one of the basic components, must be changed to those for an adaptive array antenna and it is difficult to make use of the equipment, especially the Rake receiver, employed in the wireless base station of the usual DS-CDMA mobile communications system. Consequently, the state of the art is such that the conventional method does not make possible the smooth introduction of the AAA system.
Thus, the conventional AAA system for improving the performance of the wireless base station in a DS-CDMA mobile communications system is such that almost all of the base station equipment must be replaced with AAA-compatible equipment. This has a major impact upon the cost of the wireless base station equipment and hinders the introduction of AAA systems.